The collections returned by keySet and asMap iterate
through the keys according to the key comparator ordering or the natural
ordering of the keys. Similarly, get, removeAll, and replaceValues return collections that iterate through the values according
to the value comparator ordering or the natural ordering of the values. The
collections generated by entries, keys, and values
iterate across the keys according to the above key ordering, and for each
key they iterate across the values according to the value ordering.

The multimap does not store duplicate key-value pairs. Adding a new
key-value pair equal to an existing key-value pair has no effect.

Null keys and values are permitted (provided, of course, that the
respective comparators support them). All optional multimap methods are
supported, and all returned views are modifiable.